Sugar beet has been cultivated for thousands of years as a sweets source, but its potential as a source of sugar was not discovered until the 18th century. The sugar beet is a biennial plant belonging to the Chenopodiaceae. Its usual life cycle is completed in two years. In the first year a large succulent root is developed, which serves as a reserve for energy in the form of sucrose. For this reason it is farmed as an annual. In the second year it produces flowers and seeds. If there happens to be prolonged cool periods in the first year, the seed stalk can already sprout. This genetically determined thermal induction leads to a phenomenon called bolting. Cropping the beet for sugar extraction cuts the biennial cycle in half, whilst the sucrose is at its peak.
As already mentioned an obligate part of the complete sugar beet life cycle is the cold-induced vernalization, which induces bolting of the plants. The likelihood of bolting is increased in relationship to the number of days on which the maximum temperature does not exceed 12° C. This can lead to loss of yield when the early sowing method is applied, as 1% bolters in a crop have been estimated to reduce sugar yield by 0.4-0.7%.
There exist two methods for cropping sugar beet, spring and autumn cropping, whereas they are practiced in the southern, milder, climate or in northern latitudes respectively. Both rely on varieties with different degrees of bolting resistance. Bolting resistance influences temperature, length and irradiation limits tolerable for seed stalk induction and is a key trait in sugar beet breeding. To allow for complete control of bolting and flowering, by either blocking vernalization, devernalizing vernalized plants or suppressing flower or viable seed production would allow the sugar beet crop to be sown in autumn in northern latitudes without the risk of bolting and flowering in the following season. This shift from a spring into a winter crop would permit growers to drill their crop in autumn and to harvest the next summer. Comparison of winter to spring cultivars in crops like wheat and oilseed rape has shown that winter cultivars consistently yield higher than spring crops. The result would be an improvement of the economic viability and profitability of the crop. A further advantage would be the possibility to combine the growing of spring and winter crops, which would result in an extension of the harvest campaign by starting two to three months earlier, thus allowing for the improved capitalization on investments in equipment and infrastructure necessary for sugar beet harvesting, transport and processing.
In Arabidopsis thaliana functional analysis has distinguished four distinct flowering pathways (Levy and Dean, 1998). These four pathways can be assigned to environmental stimuli, such as photoperiodic and vernalization promotion pathways, or inherent developmental signals, e.g. autonomous promotion and floral repression pathways. In some species the timing of flowering is primarily influenced by environmental factors, such as photoperiod, light quality/quantity, vernalization and water or nutrient availability. Other species are influenced less by exogenous signals and rely more on endogenous ones, such as plant size or number of nodes.
One locus of interest is the FLOWERING LOCUS C (FLC) discovered in naturally occurring late-flowering ecotypes of Arabidopsis (Koornneef et al, 1994; Lee et al. 1994). FLC is a MADS box transcriptional regulator (Michaels, S D and R M Amasino, 1999) that represses flowering.
In contrast, there are genes that cause the switch from vegetative to reproductive growth, including the “flowering locus T” (FT), “leafy” (LFY), and “suppressor of over expression of constans” referred to as “Agamous-like 20” (AGL20). (Nilsson et al, 1998; Kobayashi et al. 1999; Blazquez et al., 2000; Lee et al, 2000; Samach et al., 2000; Borner et al., 2000) Overexpression of AGL20 causes early flowering in Arabidopsis, whereas its down-regulation causes late flowering.
In the case of sugar beet, it has been shown that the vernalization response is one of the most important factors of flower induction. Although bolting resistant varieties are known and available to sugar beet farmers, still there are major problems with the cultivation of the higher yielding winter beet due to bolting incidents. Currently, there are no plants or methods for predictably delaying sugar beet vernalization.
Vernalization and its effect on biennial sugar beet have been described in detail (e.g. Jaggard et al, 1983). Sugar beet responds to temperatures between 3 and 12° C. and cooling degrees accumulate. Several weeks of 3-12° C. are required for the beet to start bolting. The ITB Bolting Model shows that in France, vernalization occurs up to 90 days (13 weeks) after drilling. Seventeen days of 7° C. is the critical number during these 90 days to initiate bolting.